Stable Isotope and Hydrochemical Evolution of Groundwater in Mining Area of the Changzhi Basin, Northern China

doi:10.21203/rs.3.rs-823775/v1

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Stable Isotope and Hydrochemical Evolution of Groundwater in Mining Area of the Changzhi Basin, Northern China

https://doi.org/10.21203/rs.3.rs-823775/v1

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The Changzhi Basin of China is an economically and ecologically important area with intensive human activities. To foster the sustainable development of groundwater resources and the economy, a total of 117 groundwater samples were collected in shallow and deep aquifers, including 91 ²H and ¹⁸O isotope samples, to improved understanding of the natural geochemical processes and the impacts of anthropogenic activities on the groundwater chemistry. Synthetical application of the stable isotopes, Piper diagram, Gibbs diagram, ionic ratios and saturation indices to data analysis led to identification of hydrochemical zones for both aquifers from west to east of the basin. Isotopic analyses suggested that the groundwater recharge mainly comes from infiltration of rain water, hydraulic interaction between surface water and shallow groundwater, and lateral recharge from fissure water at the edge of the basin. The predominant natural geochemical processes include mineral dissolution in conjunction with the cation exchange. The excess deuterium method revealed that mineral dissolution contributed 81%–98% to the salinity of shallow groundwater and 84%–98% to the salinity of deep groundwater. Anthropogenic activities are secondary contributions to the hydrochemical evolution with fertilizer application, human waste and sewage discharges causing an increase in NO₃-N content and coal mining activities affecting the ion content of Na⁺, Cl^-, SO₄^2-, and HCO₃^- in the groundwater.

Geology

Environmental Chemistry

Groundwater

Hydrogeochemistry

Stable isotopes

Hydrochemical mechanisms

Anthropogenic activities

Coal mining

Groundwater resources are increasingly exploited for industrial and agricultural purposes in many arid and semi-arid regions globally (Li et al., 2019; Liu et al., 2015). In the last few decades, rapid developments in industry, agriculture, and coal mining, together with rapid urban population growth, has led to dramatic changes in water chemistry in many developing countries (Jamshidzadeh and Mirbagheri, 2011; Sandow et.al, 2010). Water chemistry can be used to facilitate an understanding of the natural geochemical processes and the effects of anthropogenic activities (Liu et al., 2015; Selvakumar et al., 2017). The study of hydrochemical processes and the factors affecting water quality provides an insight to the controlling mechanisms of groundwater hydrochemistry, and is important for sustainable development and effective protection of groundwater (Tizro and Voudouris, 2008; Chang and Wang, 2010; Yang et al., 2016; Mohanty et al., 2019).

The combination of hydrogeochemical methods such as the Piper diagram, Gibbs diagram, ionic ratios, and multiple isotopic analyses have been widely used to study hydrochemical evolution (Plummer et al., 1990; Marfia et al., 2004; Wang et al., 2006; Qian et al., 2013; Liu et al., 2015; Sánchez et al., 2015). Isotopic and geochemical indicators often serve as effective methods for solving multiple problems in hydrology and hydrogeology, especially in semi-arid and arid regions (Clark and Fritz, 1997; Zang et al., 2018). Stable isotopes of H and O are typical tracers for investigating the sources of groundwater recharge and the hydrological cycle because the stable isotopes are constituents of the water molecules (Chen et al., 2012; Jia et al., 2017). Hydrogeochemical indicators in different water bodies can provide information on the geochemical reactions within the aquifer and on the possible evolutionary pathways of groundwater (Cook and Herczeg, 1999; Appelo and Postma, 2004).

The Changzhi Basin is located in the arid to semi-arid area of northern China. Because of population growth and the increasing development of agricultural and coal mining activities, the demand for groundwater is continually increasing. Groundwater pollution in the Changzhi Basin is, however, increasing as a result of the large-scale and long-term discharge of coal mining water, domestic sewage, and industrial waste water, and the excessive use of fertilizers. Various studies on karst groundwater in the Changzhi Basin have been conducted, and were mainly focused on karst spring flow, groundwater quality, and water resource protection (Sun, 2008; Chen et al., 2012; Yang et al., 2016). Studies on groundwater hydrochemistry and evolution processes in the porous media are lacking. The present study addresses this knowledge gap. To obtain a comprehensive understanding of the evolution of groundwater hydrochemistry in the Changzhi Basin, it is necessary to identify the impacts of natural factors and anthropogenic activities on the groundwater hydrochemistry. Consequently, the main objectives of the present study are to: (1) interpret the hydrochemical and stable isotopic characteristics; (2) ascertain the origin of groundwater and reveal the hydrochemical formation mechanisms for both natural geochemical processes and anthropogenic activities; and (3) quantify the contribution of mineral dissolution, evaporation, and cation exchange factors in controlling the chemical composition of groundwater.

The Changzhi Basin is located in the southeast of Shanxi Province (Fig. 1), encompassing an area of 1,169 km², and lying between 36°00′ and 36°19′ N and 112°46′ and 113°16′ E. The annual mean air temperature of the study area ranged from 8.9 to 10.9°C over the past 30 years. The average rainfall from 1956 to 2018 was 536 mm, with a range from 264 mm (1965) to 1,056 mm (1971). 60–70% of the annual rainfall was concentrated in July, August, and September. The annual mean water surface evaporation was 1,678 mm, with a maximum of 1,810 mm and a minimum of 1,372 mm. The Zhuozhang River, the main river in the basin, runs from the south to the north. The main tributaries of the Zhuozhang River are the Taoqing, Lanshui, Jianghe, and Yuni Rivers. The Zhangze Reservoir is the only large-scale reservoir in the study area, while there are many medium and small-sized reservoirs, such as the Baojiahe, Tunjiang, and Shencun Reservoirs.

Geologically, Changzhi Basin is a Cenozoic fault basin. The main outcropping strata in the study area are of Quaternary (Q) age. Permian (P), Carboniferous (C), and Ordovician (O) strata outcrop locally. The Quaternary sediments are 0–300 m thick, and the sediments thicken from west to east (Fig. 2). The main aquifers are in the unconsolidated sediments, and divided into shallow (≤ 50 m in depth) and deep (> 50 m in depth) aquifers. The shallow aquifer is unconfined in alluvial-proluvial sand-gravel layers of the Holocene and Upper Pleistocene. The deep aquifer is confined in silt, sand, and gravel lenses of the Middle and Lower Pleistocene. The unconfined and confined aquifers are separated by a discontinuous aquitard composed of a mudstone layer. The regional groundwater flows towards the Zhangze Reservoir and Zhuozhang River (i.e., from the west, east, and south).

3.1. Sample collection

A total of 117 groundwater samples were collected in April and November, 2018. The samples comprised 76 shallow groundwater samples, 36 deep groundwater samples, and 5 mine drainage samples. The sampling locations are shown in Fig. 1. Mine drainage samples were taken from the water drained from the mines to enable the exploitation of coal-bed methane and coal. Additionally, 24 rainwater samples were collected from central, eastern, and western areas of the basin from July, 2017 to July, 2018 for stable isotope tests; whereas, three of these samples were also used for hydrochemical tests. The samples were filtered through 0.45 µm membranes on site and then stored at 4°C. The bottles were rinsed twice with deionized water before sampling. For cation analysis, water samples were acidified using analytically pure nitric acid to pH < 2. Samples for stable isotope analysis (δ¹⁸O and δ²H) were collected in 50 mL glass bottles, which were sealed with airtight caps.

3.2. Measurement methods

Water temperature, pH, and electrical conductivity were directly measured on-site using a HANNA HI 991301 multi-parameter instrument. Major anions, cations and minor elements were analyzed by a Thermo Scientific Dionex ICS-4000 (precision = ± 1%) and PerkinElmer Optima 8300 inductively coupled plasma-optical emission spectrometer (precision = ± 1%) at the Groundwater Mineral Water and Environmental Monitoring Center in the Institute of Hydrogeology and Environmental Geology at the Chinese Academy of Geological Sciences. The analytical precision and electrical balance error of the hydrochemical data were within ± 5%.

Stable isotope ratios were expressed in δ (‰) notation and calculated with respect to Vienna Standard Mean Ocean Water (VSMOW). The δ¹⁸O and δD values in water samples were obtained using a Picarro L2130-i Analyzer at the Institute of Hydrogeology and Environmental Geology at the Chinese Academy of Geological Sciences. The analytical precision for δD was ± 1‰ and for δ¹⁸O was ± 0.1‰.

4.1. Hydrochemical characteristics

The physicochemical parameters of the water samples are shown in Table 1; all water sample data were plotted on Piper diagrams (Fig. 3a–c). A coefficient of variation (Cv) is commonly used to characterize the stability of a variable: 0 < Cv < 0.1 indicates weak variability; 0.1 < Cv < 1.0 indicates moderate variability; and Cv > 1.0 indicates wide variability (Yang et al., 2016; Zhai et al., 2019).

4.1.1. Rain water, surface water, and mine drainage

The pH values of rain water ranged from 6.39 to 6.63, indicating weakly acidic conditions. Surface water (pH 7.03–7.94) and mine drainage (pH 7.85–8.90) were generally neutral to weakly alkaline. The concentrations of the chemical components in rain water were, in general, low. Most of the chemical components in surface water and rain water exhibited a weak to medium variability, but a wide variability in mine drainage. The maximum concentrations of total dissolved solids (TDS), Na⁺, Cl⁻, and SO₄²⁻ for mine drainage were 2,901, 1,176, 1,078, and 1,218 mg·L^− 1, respectively, and were significantly greater than the maximum concentrations for surface water and rain water.

The chemical components of surface water were dominated by Ca²⁺, Na⁺, HCO₃⁻, SO₄²⁻, and Cl⁻, and the hydrochemical types were complex. The HCO₃·SO₄-Ca·Mg, HCO₃·Cl·SO₄-Ca·Na, HCO₃-Ca·Na, SO₄·Cl-Na·Ca, and HCO₃·Cl-Ca types were all identified for surface water. The chemical components of mine drainage were dominated by Na⁺, HCO₃⁻, SO₄²⁻, and Cl⁻, and hydrochemical types were mainly HCO₃·Cl-Na and SO₄-Na. The chemical components of rain water were, however, dominated by Ca²⁺, HCO₃⁻, and SO₄²⁻, and the hydrochemical type was HCO₃·SO₄-Ca.

4.1.2. Groundwater

The pH values of the shallow groundwater ranged from 7.13 to 8.07, and had a mean value of 7.53, which indicates near neutral to weakly alkaline conditions. Total hardness (TH) and TDS of the shallow groundwater varied from 168.1 to 1,889 mg·L^− 1 and from 208.8 to 2,559 mg·L^− 1, respectively. The chemical components were dominated by Ca²⁺, HCO₃⁻, SO₄²⁻, and Cl⁻. Of the shallow groundwater samples, 51% were of the HCO₃-Ca type and had a low TDS (mean value = 409.6 mg·L^− 1); 16% were of the HCO₃·SO₄-Ca/Mg type and had a medium TDS (mean value = 647.6 mg·L^− 1); and 13% were of the SO₄·HCO₃(SO₄·HCO₃·Cl)-Ca/Mg type and had a high ion content (mean value = 1,412.5 mg·L^− 1). Moreover, the SO₄·Cl-Ca, Cl·SO₄-Ca, and HCO₃·Cl-Ca type was also identified for shallow groundwater (Fig. 3b). The Cv values of K⁺, Cl⁻, SO₄²⁻, and NO₃⁻ were greater than 1.0, which indicates that these hydrochemical components had a wide variation in spatial distribution. The TDS content gradually increased (from 208.8 to 2,559 mg·L^− 1) from the west to the east of the basin. In the west of the basin, the chemical type was mainly HCO₃-Ca. However, in the east of the basin, the chemical types were complex: SO₄·HCO₃-Ca/Mg, SO₄·Cl-Ca, Cl·SO₄-Ca, and HCO₃·Cl-Ca were all identified and the TDS values were high (780–2,559 mg·L^− 1).

For deep groundwater, the pH values ranged from 7.04 to 7.97, with a mean value of 7.57, indicating near neutral to weakly alkaline conditions. The TH and TDS of the deep groundwater varied from 177.1 to 1,559 mg·L^− 1 and from 240.2 to 2,160 mg·L^− 1, respectively. The chemical components were dominated by Ca²⁺, Mg²⁺, HCO₃⁻, and SO₄²⁻. Of the deep groundwater samples, 61% were of the HCO₃-Ca/Mg type and had a low TDS (mean values = 381.8 mg·L^− 1); 17% were of the HCO₃·SO₄-Ca/Mg type and had a medium TDS (mean values = 653.45 mg·L^− 1); and 8% were of the SO₄·HCO₃-Ca/Mg type and had a high ion content (mean values = 1,277.4 mg·L^− 1). Moreover, the HCO₃·SO₄·Cl-Ca and HCO₃·Cl·SO₄-Ca types were also identified (Fig. 3c). The Cv values of Cl⁻, SO₄²⁻, and NO₃⁻ were greater than 1.0, indicating that these hydrochemical components had a wide variation in spatial distribution. The TDS of the deep groundwater gradually increased (from 240.2 to 2,160 mg·L^− 1) from the west to the east of the basin. In the west and central of the basin, the groundwater is mainly of the HCO₃-Ca type. However, in the east of the basin, as a result of low discharge and weak self-purification capacity, the SO₄·HCO₃-Ca/Mg, HCO₃·SO₄·Cl-Ca, and HCO₃·Cl·SO₄-Ca types were all identified for the deep groundwater, which had a high TDS of approximately 800–2,160 mg·L^− 1.

The concentrations of minor and trace elements, such as F, Cr, As, Fe, Mn, and Pb, were generally low, and most were not detected. In terms of the Groundwater Quality Standards of China (GB/T 14848 − 2017), values of F, Cr, and As are all below the Chinese III quality standards (1.0 mg·L^− 1 for F, 0.05 mg·L^− 1 for Cr, and 0.01 mg·L^− 1 for As). The concentrations of Fe, Mn, and Pb exceed the III quality standards in some samples. The standards for Fe, Mn, and Pb (0.3 mg·L^− 1 for Fe, 0.10 mg·L^− 1 for Mn, and 0.01 mg·L^− 1 for Pb) were exceeded in samples 6, 7, and 6, respectively. These samples were taken from shallow groundwater.

Table 1

Chemical properties and composition of groundwater, surface water, rain water, and mine drainage samples in the Changzhi Basin. n = number of samples; Cv = coefficient of variation; TH = total hardness; TDS = total dissolved solids. Units: pH in units of pH, major ions and minor elements in mg·L^− 1.
	shallow groundwater(n = 76)			deep groundwater(n = 37)			surface water(n = 11)			rain water(n = 3)			mine drainage(n = 5)
	range	average	Cv	range	average	Cv	range	Average	Cv	range	average	Cv	range	average	Cv
pH	7.13 ~ 8.07	7.53	0.02	7.04 ~ 7.97	7.57	0.03	7.03 ~ 7.94	7.42	0.04	6.39 ~ 6.63	6.47	0.02	7.85 ~ 8.9	8.41	0.05
Hardness	168.1 ~ 1889	547.58	0.68	177.1 ~ 1559	386.17	0.66	136.1 ~ 417.3	267.03	0.38	9.01 ~ 19.52	13.51	0.40	14.01 ~ 360.3	98.98	1.48
TDS	208.8 ~ 2559	744.02	0.71	240.2 ~ 2160	510.86	0.69	190.1 ~ 955	511.47	0.51	27.51 ~ 48.41	34.84	0.34	715 ~ 2901	1607.60	0.58
K⁺	0.06 ~ 30.08	1.25	3.01	0.14 ~ 2.68	0.58	0.84	1.6 ~ 17.83	7.08	0.86	0.24 ~ 0.78	0.48	0.57	1.63 ~ 22.72	6.49	1.41
Na⁺	9.08 ~ 165.7	36.67	0.80	8.9 ~ 73.72	24.62	0.64	13.2 ~ 226.2	76.34	0.88	0.39 ~ 1.24	0.75	0.59	265.3 ~ 1176	587.60	0.60
Ca²⁺	56.06 ~ 537.5	159.87	0.66	50.66 ~ 476.1	113.65	0.69	27.56 ~ 115.8	67.62	0.45	3.24 ~ 6.58	4.71	0.36	3.97 ~ 73.3	22.25	1.30
Mg²⁺	6.78 ~ 151.2	36.04	0.77	8.83 ~ 89.86	24.87	0.65	15.78 ~ 40.82	23.84	0.35	0.24 ~ 0.71	0.40	0.66	0.98 ~ 42.99	10.53	1.73
Cl⁻	7.72 ~ 548	90.28	1.26	5.26 ~ 311	40.11	1.35	13.51 ~ 196.1	80.37	0.70	1.40 ~ 2.45	1.87	0.29	76.42 ~ 1078	303.35	1.43
SO₄²⁻	5.38 ~ 720.7	144.64	1.09	6.61 ~ 469.1	90.53	1.16	48.84 ~ 314.5	127.66	0.76	6.27 ~ 11.97	8.64	0.34	2.17 ~ 1218	269.00	1.97
HCO₃⁻	157.1 ~ 581.1	305.79	0.29	197.3 ~ 482.8	272.07	0.27	94.07 ~ 290	209.70	0.33	11.72 ~ 24.16	16.85	0.39	363.7 ~ 941.8	689.46	0.37
NO₃⁻	1.84 ~ 873.4	102.83	1.37	8.39 ~ 580.9	61.59	1.65	0.2 ~ 86.66	15.42	1.72	3.61 ~ 6.51	5.41	0.29	1.9 ~ 6.67	3.82	0.48
F⁻	0.29 ~ 0.78	0.47	0.21	0.23 ~ 0.59	0.44	0.19	0.31 ~ 2.02	0.71	0.74	0.21 ~ 0.26	0.24	0.11	1.86 ~ 7.74	4.21	0.54
Cr⁶⁺	<0.004 ~ 0.01	-	-	<0.004 ~ 0.028	-	-	<0.004	-	-	<0.004	-	-	<0.004	-	-
As	<0.001 ~ 0.037	-	-	<0.001	-	-	<0.001 ~ 0.004	-	-	<0.001	-	-	<0.001 ~ 0.032	-	-
Fe	<0.01 ~ 0.058	-	-	<0.01 ~ 1.497	-	-	<0.01 ~ 0.599	-	-	<0.01	-	-	<0.01 ~ 0.09	-	-
Mn	<0.001 ~ 2.331	-	-	<0.001 ~ 0.17	-	-	<0.001 ~ 0.134	-	-	<0.001	-	-	<0.001 ~ 0.293	-	-
Pb	<0.001 ~ 0.269	-	-	<0.001 ~ 0.125	-	-	<0.001 ~ 0.058	-	-	<0.001	-	-	<0.001 ~ 0.008	-	-

Table 2

Isotopic composition of water samples taken in the Changzhi Basin.
	shallow groundwater			deep groundwater			surface water			rain water
	Min	Max	Mean	Min	Max	Mean	Min	Max	Mean	Min	Max	Mean	Min	Max	Mean
δ²H(‰)	-76	-51	-66	-74	-53	-68	-63	-46	-52	-104	-11	-45	-84	-59	-73
δ¹⁸O(‰)	-10.1	-6.3	-8.9	-9.9	-6.5	-9.1	-8.3	-5.2	-6.3	-14.1	-2.0	-6.9	-11.4	-7.9	-9.8

4.2. δ¹⁸O and δ²H characteristics and groundwater provenance

Stable water isotopes of hydrogen (δ²H) and oxygen (δ¹⁸O) are increasingly being applied as a useful tool for integrating information about hydrological processes across various scales (McDonnell and Beven, 2014; Li et al, 2019). Table 2 summarizes the isotopic composition of water samples taken in the Changzhi Basin.

4.2.1. Rain water, surface water, and mine drainage

The δ¹⁸O and δ²H of rain water, surface water, and mine drainage are plotted in Fig. 4a. The slope of the local meteoric water line (LMWL, k = 8.12) is similar to the global meteoric water line (GMWL, k = 8.0), and that the compositions of the stable hydrogen and oxygen isotopes are also aligned with the GMWL. The δ¹⁸O and δ²H compositions of rain water range from − 14.1‰ to − 2.0‰ and − 104‰ to − 11‰, respectively, surface water ranges from − 8.3‰ to − 5.2‰ and − 63‰ to − 46‰, respectively, while mine drainage samples range from − 11.4‰ to − 7.9‰ and − 84‰ to − 59‰, respectively (Table 2).

The surface water and mine drainage samples lie below the LMWL. When compared with the LMWL, the surface water and mine drainage isotopes can be seen to be enriched in heavy isotopes, and are located near the LMWL. This indicates that the surface water and mine drainage are recharged by rain water and have undergone evaporation and condensation, especially in the case of surface water. The mine drainage samples are scattered in the study area and thus could represent different recharge sources (Qian et al., 2013).

4.2.2. Groundwater

The δ¹⁸O and δ²H of groundwater are plotted in Fig. 4b. The δ¹⁸O and δ²H compositions of groundwater are located near the LMWL, which indicates that the groundwater is mainly recharged by rain water. The slope of the shallow groundwater line (k = 6.19) is smaller than that of the deep groundwater line (k = 6.41), which means that the groundwater has undergone evaporation during the recharge process, especially in the case of shallow groundwater.

The δ¹⁸O and δ²H values for shallow groundwater range from − 10.1‰ to − 6.3‰ and − 76‰ to − 51‰, respectively, while for deep groundwater the corresponding values are − 9.9‰ to − 6.5‰ and − 74‰ to − 53‰, respectively. Some shallow groundwater samples have relatively low stable isotope concentrations and some deep groundwater samples have relatively high concentrations. This may have resulted from the interaction between shallow and deep groundwater. The δ¹⁸O and δ²H values of some shallow groundwater samples are consistent with surface water, indicating an interaction between surface water and shallow groundwater. Because of the lateral recharge from fissure groundwater at the edge of the basin, the δ¹⁸O and δ²H compositions of shallow groundwater are relatively low.

4.3. Hydrochemical evolution processes

4.3.1. Effects of precipitation, evaporation, and rock-type on hydrochemistry

A qualitative analysis, using the Gibbs diagram (Gibbs, 1970), was carried out to identify the dominant processes affecting evolution. The Gibbs diagram depicts the relative dominance of precipitation, rock weathering, and evaporation in semi-arid and arid regions. The diagrams show the weight ratios of Na⁺/(Na⁺+Ca²⁺) and Cl⁻/(Cl⁻+HCO₃⁻) against TDS, as shown in Fig. 5.

Figure 5b shows that the ratios of Na⁺/(Na⁺+Ca²⁺) are mostly less than 0.5 and that the TDS is mostly low to medium, which indicates that rock weathering is the dominant mechanism in the geochemical evolution of groundwater for both shallow and deep groundwater. The ratios of Na⁺/(Na⁺+Ca²⁺), however, show a wide range of values without any obvious changes in TDS values. This suggests that cation exchange also plays an important role by increasing Na⁺ and decreasing Ca²⁺ when geochemical evolution is dominated by rock-weathering processes (Liu et al., 2015; Li et al., 2019). This occurs because 2 mmol·L^− 1 of Na⁺ is exchanged with 1mmol·L^− 1 of Ca²⁺ during the cation exchange, and the mass concentration of 2 mmol·L^− 1 of Na⁺ (46 mg·L^− 1) is nearly equal to 1mmol·L^− 1 of Ca²⁺ (40 mg·L^− 1). Some samples of shallow groundwater, however, are located on the upper right of the Gibbs diagram (Fig. 5a), and the ratios of Cl⁻/(Cl⁻+HCO₃⁻) are greater than 0.5 and the TDS value greater than 1,100 mg·L^− 1. This shows that the groundwater chemistry is controlled not only by rock weathering, but also by evaporation (Xing et al., 2013).

4.3.2. Effects of evaporation on groundwater salinity based on stable isotope

The deuterium excess, as one of the most important indicators characterizing atmospheric precipitation for comprehensive environmental factors, is capable of quantifying the contribution of evaporation to groundwater salinity (Dansgaard, 1964; Li et al., 2019; Huang and Pang, 2012). In the study area, δ²H₀ and δ¹⁸O₀ were calculated for − 45‰ and − 6.9‰ of local mean rain water, although the δ²H₀ and δ¹⁸O₀ values do not affect the mineral dissolution to total salinity relationship; ${S}_{0}$, the total salinity of mean rain water, was defined as 0.035 g·L^− 1; f is the remaining fraction of the reservoir; the average temperature of the groundwater was 14°C and the humidity was about 60%. Thus, the relationship between $d$ and $f$ can be established as given in Eq. (1) (Huang and Pang, 2012).

$$d=\delta {}^{2}H-8\delta {}^{18}O=\left({\delta }^{2}{H}_{0}+1000\right){f}^{({\alpha }^{2}{H}_{0}-1)}-8\left({\delta }^{18}{O}_{0}+1000\right){f}^{\left({\alpha }^{18}{O}_{0}-1\right)}+7000$$

$$=955\times {f}^{-0.08676}-8\times 993.1\times {f}^{-0.01546}+7000 \left(1\right)$$

$\frac{{S}_{0}}{f}-{S}_{0}$ was the salinity caused by direct evaporation; D was the salinity increased by mineral dissolution, and $D=S-{S}_{0}-(\frac{{S}_{0}}{f}-{S}_{0})$ (Huang and Pang, 2012).

As shown in Fig. 6a, the average total d-excess and TDS for shallow groundwater and deep groundwater are 4.63‰ and 744.02 mg·L^− 1, 4.72‰ and 510.86 mg·L^− 1, respectively. The contribution ratios of mineral dissolution in shallow groundwater and deep groundwater are 81–98% and 84–98%, while the contribution ratio of evaporation is 0.2–4.7% and 0–2.4%. In the study area, most groundwater depths are greater than 5 m, and the groundwater is mainly recharged by rain water and bedrock fissure water, which favors the dissolution of minerals. Most of the water samples indicate no evaporation effects regardless of shallow groundwater or deep groundwater. Figure 6b and 6c show that there is an almost exponential positive correlation between TDS and the contribution of mineral dissolution, and an almost exponential negative correlation between TDS and the contribution of evaporation, which indicates that mineral dissolution is the main contributor to the total salinity of groundwater.

4.3.3. Effects of geochemical processes on hydrochemistry

The relationship between (Ca²⁺ + Mg²⁺) and (HCO₃⁻ + SO₄²⁻) concentrations in groundwater samples is close to the carbonate and gypsum dissolution line (1:1 relationship line) if these ions are controlled by carbonate and gypsum equilibrium (Wang et al., 2006; Zhang et al., 2015). Ion exchange (Ca²⁺ and Mg²⁺ in groundwater changed by Na⁺) tends to shift points to the right in equilibrium plots as a result of an excess of (HCO₃⁻ + SO₄²⁻) (Belkhiri et al., 2011; Liu et al., 2015), but reverse ion exchange shifts the points to the left as a result of an excess of (Ca²⁺+Mg²⁺). As shown in Fig. 7a, values for most of the deep groundwater samples plot near the 1:1 line, which indicates that the dissolution of carbonate and gypsum is the main geochemical process in deep groundwater. Values for most of shallow groundwater samples are, however, scattered and located above the 1:1 line. This suggests that ion exchange is one of the main geochemical processes occurring in shallow groundwater in addition to the dissolution of carbonate and gypsum.

As shown in Fig. 7b, in the shallow groundwater, the Na⁺/Cl⁻ mole ratio varies from 0.37 to 13.27, with an average value of 2.07. In the deep groundwater, the ratio ranges from 0.18 to 7.24, with an average value of 1.21. Most of the values for the deep groundwater samples are located near the 1:1 line, which suggests that the dissolution of halite is the major source of Na⁺ and Cl⁻ for deep groundwater. In contrast, most of the shallow groundwater values are scattered and are located below the 1:1 line. This indicates that there are other geochemical processes than halite dissolution leading to an excess of Cl⁻, such as Ca(Mg)/Na ion exchange.

Ion exchange and adsorption are common reactions in the geochemical evolution of groundwater and influence the major ion composition of groundwater (Li et al., 2019). Schoeller (1967) proposed the chloral-alkali index (CAI) to analyze cation exchange and adsorption (Eqs. (2) and (3)).

$$CAI1=\frac{{Cl}^{-}-({Na}^{+}+{K}^{+})}{{Cl}^{-}} \left(2\right)$$

$$CAI2=\frac{{Cl}^{-}-({Na}^{+}+{K}^{+})}{{SO}_{4}^{2-}+{HCO}_{3}^{-}+{CO}_{3}^{2-}+{NO}_{3}^{-}} \left(3\right)$$

A positive CAI indicates the exchange of Ca²⁺ and Mg²⁺ from the rocks with Na⁺ of the water, while a negative CAI indicates the exchange of Na⁺ from the rocks with Ca²⁺ and Mg²⁺ of the water. Additionally, a diagram of [(Ca²⁺+Mg²⁺) − (HCO₃⁻+SO₄²⁻)] versus (Na⁺−Cl⁻) (Eq. (4)) is also commonly used to explain cation exchange in groundwater (Carol et al., 2013; Huang et al., 2013; Liu et al., 2015). The diagram reflects excess Ca²⁺ and Mg²⁺ gained or lost from calcite, dolomite, and gypsum dissolution or precipitation, and excess Na⁺ gained or lost from NaCl (Farid et al., 2013). If cation exchange plays a major role in controlling the major ion composition of groundwater, there will be a linear relationship with a slope close to − 1.0, as expressed by Eq. (4).

$$N=\frac{\left({Na}^{+}+{K}^{+}\right)-{Cl}^{-}}{{Ca}^{2+}+{Mg}^{2+}-{(HCO}_{3}^{-}+{SO}_{4}^{2-})} \left(4\right)$$

Figure 8a shows that both positive and negative ion exchange occurs in groundwater. For shallow groundwater, the positive ion exchange was slightly dominant, and accounted for 51.3% of the ion exchange. This process mainly occurred at the central of the Changzhi Basin, which have a slower runoff and more shallow groundwater compared with the edge of the basin, leads to the large Na⁺ content in the water displaced part of Ca²⁺ in the aquifer. In the edge part of the basin, the cations in the groundwater are dominated by Ca²⁺ and Mg²⁺. The groundwater passes through rocks that contain Na-rich minerals, which leads to exchange reactions and increased Na⁺ in groundwater. For deep groundwater, the negative ion exchange process dominates, and accounts for 66.7% of the ion exchange. This process mainly occurs in the edge part of the basin, and is consistent with the shallow groundwater processes. The CAI1 and CAI2 of shallow groundwater ranged from − 6.3 to 0.81 and − 0.24 to 0.76, respectively, while the CAI1 and CAI2 of deep groundwater ranged from − 12.4 to 0.62 and − 0.31 to 0.22, respectively. This suggested that the intensity of the cation exchange is different for the shallow and deep groundwater.

The slope and correlation coefficients of the equations for the shallow and deep groundwater (Fig. 8b) are − 1.88 (R² = 0.86) and − 0.34 (R² = 0.88), indicating that Ca²⁺, Mg²⁺, and Na⁺ participate in ion exchange. However, significant differences were observed between the theoretical and actual values, implying that cation exchange is not the sole process affecting the concentration of the three ions. The other processes affecting the ion content include the discharge of mine drainage, and the interaction between groundwater and surface water. These processes affect ion content because of the high Na⁺ content of mine drainage and surface water (Table 1).

To better understand the hydrogeochemical processes in the aquifers, PHREEQC (Parkhurst and Appelo, 1999) was used to calculate the saturation indices of the major minerals. The saturation indices of minerals varied between − 0.5 and + 0.5, which indicates that groundwater is saturated (or in equilibrium) or near saturation with respect to these minerals. As shown in Fig. 9a and b, most of the groundwater samples are in a state of saturation or over-saturation with respect to calcite and dolomite. Almost all the groundwater samples are in a state of under-saturation with respect to gypsum and are highly unsaturated in terms of halite (Fig. 9c and d). Precipitation is the main source of groundwater in the study area. During the percolation of weakly acidic rain, carbonate minerals dissolve quickly and it is easy for groundwater to reach a dissolution equilibrium with calcite and dolomite. No significant correlation is observed between TDS and the SI values of calcite and dolomite. SI values of gypsum and halite, however, tend to increase with TDS, which indicates that the dissolution of gypsum and halite is one of the main processes involved in the increase in groundwater salinity.

4.4. Effects of anthropogenic activities on hydrochemistry

The Changzhi Basin has a long history of agricultural development, and the main crops grown are corn and wheat. Furthermore, the Changzhi Basin is located in southeastern Shanxi Province, near the location of the Jindong coal-based industries. Human activities associated with these socio-economic developments impose extensive impacts on the groundwater environment. These impacts stem from the use of fertilizers in the agricultural areas, water drainage during coal mining, and sewage discharge from urban areas.

Groundwater pollution caused by anthropogenic activities is a world-wide issue (Li et al., 2019). According to the monitoring results from 195 cities in China, 97% of urban groundwater has been polluted (Zhang, 2015). From Table 1, the following are observed of the NO₃⁻ concentrations:

22 of the shallow groundwater samples exceed the limits of the level III quality standards for groundwater (20 mg·L^− 1 as NO₃-N), and have a mean value of 55.3 mg·L^− 1.
18 of the shallow groundwater samples exceed the limits of the level IV quality standards for groundwater (30 mg·L^− 1 as NO₃-N) and have a mean value of 65.68 mg·L^− 1.
Seven of the deep groundwater samples exceed the limits of the level III quality standards for groundwater and have a mean value of 47.44 mg·L^− 1.
Three of the deep groundwater samples exceed the limits of the level IV quality standards for groundwater, and have a mean value of 76.28 mg·L^− 1.

Thus, NO₃-N pollution in the shallow groundwater is more serious than that in the deep groundwater, and mainly occurs in the central and northeastern agricultural areas, and in the urban area in the east of the basin. The main sources of NO₃-N exceedance are fertilizer application and the discharge of human waste and sewage. The average nitrogen fertilizer application rate per ha of cultivated land is approximately 832.81 kg, which is far in excess of the average rate for China (339 kg/ha) (Chen et al., 2016).

Mine drainage during coal mining is also a cause for concern because of the relatively high Na⁺, Cl⁻, SO₄²⁻, HCO₃⁻, and TDS concentrations. When compared with the combined average values for shallow groundwater, deep groundwater, and surface water, the mine water has the following characteristics:

Mean Na⁺ content of mine drainage was 7.69–23.87 times that of the combined average.
Mean Cl^- content was 3.36–7.56 times the average.
Mean SO₄^2- content was 1.86–2.97 times the average.
Mean HCO₃^- content was 2.25–3.29 times the average.
Mean TDS content was 2.16–3.15 times the average.

The amount of mine drainage was 0.39 million m³·d^− 1 in the Changzhi mining area, and was directly discharged into surface water, which interacts with groundwater, especially shallow groundwater, thus leading to an increase in Na⁺, Cl⁻, SO₄²⁻, and HCO₃⁻ content of groundwater.

In this study, the integrated approach consisting of Piper diagram, stable isotopes, Gibbs diagrams and ionic ratios provided an efficient way for analyzing the groundwater origin and hydrochemical processes that affected water chemistry. The Piper diagram and coefficient of variation were used to characterize the groundwater hydrochemistry and the stability of ions content; stable isotopes was a useful tool for analysis the origin and transformation of groundwater; Gibbs diagrams were used to establish the dominant effects of precipitation, rock weathering, or evaporation on geochemical evolution of groundwater, and deuterium excess was a capable way to quantify the contribution of evaporation to groundwater salinity; ionic ratios and saturation indices were used to depict the effects of mineral dissolution or precipitation on groundwater salinity. These methods were complementary of and verify each other. The main conclusions drawn are summarized as follows:

The groundwater chemistry type of both shallow and deep groundwater demonstrates zonational characteristics from the west to east of the basin; the TDS content gradually increases (from 208.8 to 2,559 mg·L^− 1) and the types of hydrochemistry tend to be complex. In both shallow and deep groundwater, the hydrochemistry types are mainly HCO₃-Ca and HCO₃·SO₄-Ca·Mg. The stable isotope compositions suggest that rain water is main recharge source for both shallow and deep groundwater. The interactions between shallow and deep groundwater, surface water and shallow groundwater and the lateral recharge from fissure groundwater at the edge of the basin have affected the isotopic composition of groundwater.

The hydrochemical and isotopic interpretation showed that the hydrochemical composition of the groundwater was controlled by geochemical processes. Gibbs diagrams suggested that water-rock interaction was the main mechanism controlling groundwater chemistry. The deuterium excess method revealed that that mineral dissolution accounts for 81–98% of the salinity of shallow groundwater and 84–98% of deep groundwater. The dissolution of gypsum and halite makes a significant contribution to the increase of groundwater salinity. Overall, rock weathering in conjunction with the cation exchange absolutely predominated in the geochemical evolution of groundwater.

The hydrochemical composition of groundwater in the study area is also affected by anthropogenic activities. NO₃-N pollution occurs in the central area, the northeastern agricultural area, and the urban area in the east of the basin, and is more serious for shallow groundwater than for deep groundwater. The main sources of NO₃-N are fertilizer application, human waste, and sewage. Mine drainage has relatively high concentrations of Na⁺, Cl⁻, SO₄²⁻, HCO₃⁻, and TDS when compared with shallow groundwater, deep groundwater, and surface water. Mine drainage is directly discharged into surface water and consequently interacts with groundwater, thus leading to an increase of Na⁺, Cl⁻, SO₄²⁻, and HCO₃⁻ content of groundwater. The results of the present study provide a deeper insight into the water quality situation and geochemical evolution of groundwater, and will assist decision-makers to formulate sustainable groundwater management strategies for the study area.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgments

This work was supported by the Fundamental Research Funds for the Chinese Academy of Geosciences (No. JYYWF20180401) and the China Geological Survey project (No. DD20160296, DD20201123). We thank Paul Seward, PhD, from Liwen Bianji, Edanz Editing China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript.

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You are reading this latest preprint version

Stable Isotope and Hydrochemical Evolution of Groundwater in Mining Area of the Changzhi Basin, Northern China

Status:

Version 1

Abstract

Figures

1. Introduction

2. Regional Hydrogeology

3. Sampling And Analysis

3.1. Sample collection

3.2. Measurement methods

4. Results And Discussion

4.1. Hydrochemical characteristics

4.1.1. Rain water, surface water, and mine drainage

4.1.2. Groundwater

4.2. δ¹⁸O and δ²H characteristics and groundwater provenance

4.2.1. Rain water, surface water, and mine drainage

4.2.2. Groundwater

4.3. Hydrochemical evolution processes

4.3.1. Effects of precipitation, evaporation, and rock-type on hydrochemistry

4.3.2. Effects of evaporation on groundwater salinity based on stable isotope

4.3.3. Effects of geochemical processes on hydrochemistry

4.4. Effects of anthropogenic activities on hydrochemistry

5. Conclusions

Declarations

Conflicts of interest

Acknowledgments

References

Status:

Version 1

Stable Isotope and Hydrochemical Evolution of Groundwater in Mining Area of the Changzhi Basin, Northern China

Status:

Version 1

Abstract

Figures

1. Introduction

2. Regional Hydrogeology

3. Sampling And Analysis

3.1. Sample collection

3.2. Measurement methods

4. Results And Discussion

4.1. Hydrochemical characteristics

4.1.1. Rain water, surface water, and mine drainage

4.1.2. Groundwater

4.2. δ18O and δ2H characteristics and groundwater provenance

4.2.1. Rain water, surface water, and mine drainage

4.2.2. Groundwater

4.3. Hydrochemical evolution processes

4.3.1. Effects of precipitation, evaporation, and rock-type on hydrochemistry

4.3.2. Effects of evaporation on groundwater salinity based on stable isotope

4.3.3. Effects of geochemical processes on hydrochemistry

4.4. Effects of anthropogenic activities on hydrochemistry

5. Conclusions

Declarations

Conflicts of interest

Acknowledgments

References

Status:

Version 1

4.2. δ¹⁸O and δ²H characteristics and groundwater provenance